Method and apparatus for peening with liquid propelled shot

ABSTRACT

Systems and methods for generating beneficial residual stresses in a material by impacting the surface of the material with particles that are softer than the material to be peened (“target material”). Shock waves emanate through the target material from the soft particle impacts to generate residual stresses without significantly deforming the surface of the target material. A high pressure liquid is accelerated through a peening nozzle to generate a high-speed liquid jet that is used to accelerate the soft particles that impact the surface of the target material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 61/477,255, filed Apr. 20, 2011, entitled “Method and Apparatus for Peening With Liquid Propelled Shot,” which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to systems and methods for generating beneficial residual stresses in materials by impacting the surface of the materials with particles that are softer than the material to be peened (“target material”). Shock waves emanate through the target material from the soft particle impacts to generate residual stresses without significantly deforming the surface of the target material.

BACKGROUND OF THE INVENTION

The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

Peening is the process of inducing residual compressive stresses in materials in order to improve properties such as fatigue resistance and stress corrosion cracking resistance, or to shape a part through peen forming (e.g., aircraft wing skins). The most common conventional method is to blast the surface of a material to be peened with small particles (referred to as “shot”) which are normally harder than the material to be peened. Shot peening methods that utilize gas to propel hard shot into metal materials to perform peening are known. (See e.g., U.S. Pat. Nos. 7,699,449; 6,153,023; and 4,365,493).

The shot is typically propelled with compressed air using automated equipment to move a peening nozzle over the surface of the part or material to be peened. The shot, frequently steel or ceramic, is usually accelerated to 50-100 meters per second (m/s) by the compressed air and strikes the material with enough energy to deform the surface beyond its elastic limit (i.e., plastic deformation). The deformed surface material yields permanently and creates local compressive residual stresses that result in improved fatigue life of the peened part, and can improve resistance to stress corrosion cracking, or beneficially change the shape of the part.

Variations on this method include striking the surface with particles spun off from a rotating wheel (see U.S. Pat. No. 3,834,200), low plasticity burnishing with a ball that is hydraulically pressed into the surface as it rolls across the material, and vibrating captured balls against the surface, also called ultrasonic peening (see U.S. Pat. No. 7,276,824).

These methods all involve plastic deformation of the top layer of the target material to generate residual compressive stresses. This plastically deformed surface is critical to inducing residual compressive stresses in the material since the material underneath the surface, which is not plastically deformed, tries to “push” the plastically deformed material back into its original volume. This “pushing” is the compressive stress that is beneficial for fatigue resistance.

More recently developed alternative methods include laser shock peening (see U.S. Pat. No. 5,932,120) and cavitation peening (see U.S. Pat. No. 7,716,961). These processes work not by deforming the target surface with shot or balls to produce a plastic layer, but by inducing a shock wave that travels through the material to be peened. The magnitude of the shock wave is sufficient to exceed the dynamic yield strength of the material, thereby inducing deeper residual compressive stresses than can be produced by conventional shot peening.

Biopeening uses hard particles to embed biocompatible materials into a target surface to encourage tissue attachment (see U.S. Pat. No. 6,502,442).

Additionally, U.S. Pat. No. 6,153,023 discloses a method where the shot hardness can be as low as 80% of the Vicker's Hardness of the target material.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in the referenced figures. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 illustrates a schematic diagram of a peening system according to an embodiment of the present invention.

FIG. 2 illustrates a soft particle feed hopper and control valve operative to regulate the flow of soft particles to a peening head of the peening system of FIG. 1.

FIG. 3 illustrates an embodiment of the peening head of the peening system of FIG. 1.

FIG. 4 illustrates a graph of residual stress versus depth profile for titanium material peened with conventional shot and titanium material peened with soft shot.

FIG. 5 illustrates a graph of residual stress versus depth profile for stainless steel material peened with soft shot.

FIG. 6 illustrates a graph of residual stress versus depth profile for titanium material peened with soft shot and carburized 9310 alloy steel peened with soft shot.

DESCRIPTION OF THE INVENTION

One skilled in the art will recognize many methods, systems, and materials similar or equivalent to those described herein, which could be used in the practice of the present invention. Indeed, the present invention is in no way limited to the methods, systems, and materials described.

The inventors of the present invention have recognized that all of the aforementioned methods have various shortcomings and limitations. Some or all of these shortcomings and limitations are remedied by the embodiments of the present invention discussed below. What follows is a discussion of some of the recognized shortcomings of past peening methods.

Conventional shot peening only produces shallow compressive stresses, typically less than 0.0098 inches (0.25 mm) deep. It also has the considerable drawback of roughening up the target surface, thereby causing a limitation to the improvement in fatigue life. The rough surface can also provide initiation sites for pitting and other forms of corrosion.

Further, using conventional shot peening, it is impossible to peen into tight corners of the target material because the shot size must be small to reach small radii corners. However, shot having this small of a size is typically ineffective because it does not have enough energy to plastically deform the target material.

One fundamental limitation of the conventional peening method is that the shot is accelerated using air, so it cannot reach very high velocities. Momentum transfer from the air to the shot particle is inversely proportional to the ratio of the densities of the air and the shot particle. Because the shot particles are more than 5,000 times as dense as the air, shot particles generally are not accelerated above 150 m/s and are typically accelerated much slower than 150 m/s.

Another fundamental limitation is that the conventional peening process relies on plastic deformation of the surface, so the shot particles must be harder than the target material or very nearly as hard and the target surface is inevitably roughened by the peening process. Yet another limitation of conventional shot peening is that the beneficial residual stresses can easily be relieved by exposure to high operating temperatures, or even by cyclic loading. Another limitation is that the surface magnitude of the residual stress for hard, high strength materials like carburized steel is only about 50-60% of the yield strength.

Low plasticity burnishing is limited to accessible geometry that will allow access to the rolling ball and hydraulic actuators. Ultrasonic peening is faced with almost identical limitations.

Laser shock peening is comparatively slow and very expensive. The equipment costs millions of dollars per station. Additionally, good line-of-sight is necessary so peening holes, grooves, remote locations, or tube insides is very difficult. Thus, the process is normally only used for high value applications that can justify the high cost, such as some critical aircraft engine components and in-situ nuclear reactor weldments.

Cavitation peening is lower cost that laser shock peening but is more expensive than conventional peening. Residual stresses are deeper than conventional peening. However, cavitation peening must be performed submerged in a liquid.

As noted above, U.S. Pat. No. 6,502,442 discusses biopeening, suggesting peening with garnet, where the goal is not embedment but generation of residual stress. However, the hard particles roughen the surface and can become embedded in the target material. The garnet will also erode the material, machining away the impact surface and severely limiting the effectiveness of the peening effect. The resulting residual stresses are typically very shallow. Additionally, the garnet is much harder than the target material.

Waterjet peening without any particles uses high pressure to generate fast moving droplets that impinge on the target surface. Some beneficial peening effect has been observed, but surface erosion is an issue. Further, higher magnitude residual stresses at greater depths would be desirable but are not achievable using waterjet peening.

Embodiments of the present invention are directed systems and methods for generating improved residual stress profiles in target materials without significant surface distortion in a range of materials, such as metals and ceramics. In some embodiments, a high pressure liquid is accelerated through a peening nozzle to generate a high-velocity liquid jet that is used to accelerate particles or shot that impact the surface of a target material. In some embodiments, the shot or particles are significantly softer than the material being peened, and may be referred to herein as “soft shot” or “soft particles.” Generally, the soft shot or particles may have a Vicker's Hardness value that is less than or equal to 75% of the Vicker's Hardness value of the target material. These novel systems and methods rely only on particle impact shock waves to generate residual stresses and not significant material deformation.

When the accelerated soft particles impact the surface of the target material, the resulting shock waves are capable of generating beneficial residual stresses in the material. The soft particles are accelerated to a very high speed such that they impact the surface of the target material in a way that shock waves are generated without damaging or significantly deforming the surface of the material. The shock waves from the soft particle impacts on the target material surface result in a beneficial residual stress/depth profile that is superior over past practice shot peening. The resulting stress/depth profile is similar to those produced using laser shock peening or low plasticity burnishing, but without the complex implementation, high cost, and deployment limitations. The embodiments of the present invention are capable of peening a range of materials, including metals, ceramics, and polymers.

FIG. 1 is a schematic block diagram of a peening system 10 in accordance with an embodiment of the present invention. The system 10 comprises a soft particle feed hopper or container 32 configured for storing soft peening shot or particles 50, which pass through a control valve 34 that regulates their flow into a rigid or flexible conduit 36 that conducts the flow of the soft particles 50 to a peening head 22. The soft particles 50 may be made from a wide variety of materials, such as, but not limited to, metals (e.g., annealed copper, lead, aluminum, brass, etc.), polymers (e.g., rubber, acrylic, Viton®, polyethylene, etc.), organic materials (e.g., nut shells or corn husks), or other materials.

A high pressure liquid pump 26 is provided to generate liquid pressures that are preferably 10,000 psi to 60,000 psi, up to 100,000 psi, or higher. A rigid or flexible high pressure liquid conduit 28 is used to couple pressurized liquid 64 from the pump 26 to the peening head 22. The liquid 64 may comprise liquid water, cryogenic liquid, or other suitable liquid. As an example, the pump 26 may be a KMT Waterjet Streamline V, a Flow International 20× pump, or other suitable pumps.

The peening head 22 (or a plurality of peening heads) is mounted to a robotic manipulator 14 configured to provide relative motion between the peening head 22 and a target material 40 (e.g., the portion thereof to be peened). The relative motion is designed such that the peening particles 50 strike a surface 42 of the target material 40 in areas that are desired to be peened. The robotic manipulator 14 may be coupled to a computer control unit 18 configured to preprogram and control the movement of the peening head 22 in a plurality of dimensions and to control the starting and stopping of the peening process (e.g., by controlling the operation of the control valve 34, pump 26, etc.) using pre-programmed instructions. Alternatively, the target material 40 may be mounted on the robotic manipulator 14 to provide the relative motion with the peening head 22 being stationary. A further alternative is that both the peening head 22 and the target material 40 are mounted on separate robotic manipulators 14 to provide the relative motion. Additionally, the peening head 22 could also be held by a person and a peening nozzle 68 (see FIG. 3) of the peening head 22 could be pointed at the surface 42 of the target material 40, the operator manually moving the peening nozzle 68 to peen the area of the material that is to be peened. As an example, the robotic manipulator 14 may be a Flying Bridge available from Flow International, a PAR Vector CNC, or other suitable robotic manipulator. An additional alternative is that, if only a small area is to be peened in one operation, peening may be performed with no relative motion between the nozzle 68 and target material 40.

FIG. 2 is a detailed view of the soft particle feed hopper 32 and control valve 34 that are used to supply and regulate the mass flow rate of the soft particles 50 to the peening head 22 via the conduit 36. The control valve 34 comprises an on/off valve portion 34A and a variable flow control portion 34B configured to selectively control the mass flow rate of the soft particles 50.

FIG. 3 illustrates a detailed view of the peening head 22 according to one embodiment. As shown, the peening head 22 comprises a main body 54 having a liquid input port 56 and a soft particle input port 60. The high pressure liquid 64 is fed to the liquid input port 56 at the top of the main body 54 of the peening head 22 and passes through a liquid nozzle 58, where it is accelerated to form a high velocity liquid jet 62. The high velocity liquid jet 62 then passes through a mixing or induction chamber 66 and an output or acceleration nozzle 68, creating a vacuum that operates to draw the soft particles 50 into the mixing chamber 66 of the main body 54 where the energy of the high speed jet 62 is transmitted to the particles 50 in the acceleration nozzle 68 to form an accelerated liquid/shot jet 70. The particles 50 are accelerated to high velocities within the acceleration nozzle 68 and emerge from the end of the nozzle at a high enough velocity that they are capable of generating beneficial residual stresses in the target material 40. A typical velocity range for the particles 50 may be above 300 m/s (e.g., 300 to 1,000 m/s, 500 to 1,000 m/s, 600 to 1000 m/s, or greater). A stand-off distance D between the end of the acceleration nozzle 68 and the surface 42 of the target material 40 is maintained while peening. The stand-off distance D may range between approximately ⅛ of an inch to 15 inches, for example.

In some embodiments, instead of feeding the particles 50 to the peening head 22 via the dry soft particle feed hopper 32, the particles 50 may be first mixed with a carrier liquid to form a slurry and the slurry may be fed into the peening head 22 by a slurry feed system. In other embodiments, the particles 50 may be fed to the peening head 22 by a pressurized feed system (e.g., a pressure pot). It should be appreciated that other methods may be used to feed the particles 50 to the peening head 22.

FIGS. 4, 5, and 6 illustrate graphical data depicting examples of the types of compressive residual stresses (measured in kilopounds per square inch (KSI)) that may be generated by use of the embodiments of the present invention. The residual stresses may be tailored to be higher or lower stress levels and depths within the target material 40 than those shown in the Figures, depending upon the selected operating parameters. The stress magnitudes and residual stress depths that are generated can be higher, and therefore more beneficial in many cases, than possible by past practice shot peening methods.

More specifically, FIG. 4 illustrates a graph 80 of residual stress (in KSI) versus depth profile for a titanium target material peened with conventional shot (line 84) and titanium material peened with the soft shot 50 (line 88). As shown, the titanium material peened with the soft shot 50 has more residual stress at greater depths than the titanium material peened with conventional shot. FIG. 5 illustrates a graph 90 of residual stress versus depth profile for stainless steel material peened with the soft shot 50 (line 94). FIG. 6 illustrates a graph 100 of residual stress versus depth profile for titanium material peened with the soft shot 50 (line 104) and carburized 9310 alloy steel peened with the soft shot 50 (line 106).

Embodiments of the present invention overcome the difficulties of past practices by using pressurized liquid 64 to accelerate soft shot 50 or soft particles to very high velocities. Instead of deforming the surface 42 of the target material 40 using hard shot as occurs with conventional shot peening, the fast moving soft particles 50 impact the surface 42 and induce a shock wave in the material 40 without significantly indenting or deforming the surface. The shock waves generated by the soft particles 50 travel into the material 40, exceeding the dynamic yield strength to some depth, thereby inducing relatively deep residual compressive stresses. Because the shot material 50 is much softer than the target material 40, the shot material does not embed in the target material or cause roughening of its surface 42. Further, because the density of the liquid 64 is roughly 1,000 times the density of air, it is much more efficient at accelerating the particles 50 than air-propelled methods. Because the liquid 64 is at a very high pressure, the fluid velocity is very high—high enough to accelerate the particles 50 to sufficient velocity to induce shock waves in the target material 40. Because the particles 50 are moving faster than particles move in conventional shot peening, they can be much smaller, allowing access to tight corners and other stress concentration areas. In other cases where large areas are to be peened, relatively large particles can be used efficiently if desired due to the large amount of energy available in the liquid jet. For example, the soft particles 50 may range in size from 0.044 mm to 12 mm, but generally are no larger than a fraction (no larger than roughly ½) of the diameter of the bore of the acceleration nozzle 68. In some embodiments, the particles 50 may range in size between 0.025 mm and 1 mm when using a 2 mm inside diameter acceleration nozzle 68. Alternatively, particles 50 may range in size from 0.025 mm to 6 mm when using a 12 mm inside diameter acceleration nozzle 68. As discussed below, in other embodiments the particles 50 may be other sizes. Additionally, the acceleration nozzle 68 may be other sizes than those discussed in the preceding discussion.

Because the system relies on shock waves, rather than surface deformation, the residual stresses are much deeper than conventional shot peening and are comparable to laser shock peening. Because there is little or no roughening of the surface 42 of the target material 40, there is reduced generation of pit initiation sites. Thus, surfaces intended for bearings or sealing, which require a good surface finish, can be peened without significant roughening. Moreover, fatigue life is enhanced through deeper residual stress and a smoother surface. Effective surface coverage rates are even higher than conventional shot peening, so costs can be kept very low. Additionally, the soft particles 50 are not as brittle as conventional shot, so they do not break down during peening and can be reused many times, thereby further reducing costs.

The process parameters that result in successful peening can vary widely, depending on the desired results. The liquid pressure is preferably between 10,000 and 150,000 psi or higher. In some embodiments, the liquid pressure is selected between 10,000 and 60,000 psi, while other embodiments may be operated using liquid pressures between 40,000 and 90,000 psi, and other embodiments may operate using liquid pressures between 60,000 and 125,000 psi or higher. The size range for the liquid nozzle 58 (see FIG. 3) may be between 0.005 and 0.060 inches in diameter or larger, however larger liquid nozzles require more pumping horsepower and may limit the pressure that can be used, depending upon the pump that is available. The diameter of the accelerating nozzle 68 may range from roughly 0.020 inches to 1 inch in diameter, but is generally several times the diameter of the liquid nozzle 58. The soft particles 50 may range in size from 0.044 mm to 12 mm), but generally are no larger than a fraction (e.g., no larger than ½) of the diameter of the bore of the acceleration nozzle 68. A range of stand-off distances D between the end of the accelerating nozzle 68 and the surface 42 of the target material 40 is possible. For example, a stand-off distance range of roughly ⅛ to 15 inches may be used. The traverse speed of the liquid/shot jet 70 over the target material 40 may be set to roughly 10 to 600 inches per minute, depending on the type of the target material, the desired stress intensity, and the other operating conditions. The resulting peening intensity may vary, depending on the type of the target material 40 and the peening operating parameters, but 180,000 psi and higher have been demonstrated with surface stress. As discussed above, the soft particles 50 may be made from a wide variety of materials, such as, but not limited to, metals (e.g., annealed copper, lead, aluminum, brass, etc.), polymers (e.g., rubber, acrylic, Viton®, polyethylene, etc.), organic materials (e.g., nut shells or corn husks), or other materials.

Generally, the selected operating parameters depend on size constraints caused by limited access to the target material 40, the shape of the target material, the type of the target material, limitations in abrasive material compatibility, and desired residual stress results. The example particle materials described above may have a range of Durometer 30 Shore A hardness (e.g., for softer rubbers) to 65 Shore D (e.g., for polyurethane), to 3 Mohs for walnut shells, and to Rockwell B77 for brass particles. Generally, as can be appreciated, there is no set preferred hardness, because the selected particle material depends on the hardness of the target material 40, desired residual stress profile, and desired surface finish the target material after peening.

As an example, in one embodiment of the present invention, the soft particles 50 used are the polymer Acrylic with a hardness of approximately 3.5 Mohs and size of 80 mesh (0.177 mm) travelling at a velocity of approximately 800 m/s, where the accelerating fluid is water pumped at 420 MPa (60,000 psi). The target material 40 may be hardened steel in this example, having a Mohs equivalent hardness of 6. However, it should be recognized that in some embodiments, the soft particles 50 are so much softer than the target material 40 their hardness should be measured on different hardness scales. For example, polymers may be measured on a Durometer scale and metals may be measured on a Rockwell or Brinell scale.

It should be appreciated that the size of the particles 50 may vary depending on the diameter of the acceleration nozzle 68. For example, particles 50 having a diameter of 0.25 mm may work well with an acceleration nozzle 68 having an inside diameter a 0.5 mm or larger. Similarly, a particle size of 0.841 mm may work well with an acceleration nozzle 68 having an inside diameter of approximately 1.5 mm or larger. In some embodiments, the acceleration nozzle 68 may be larger in size (e.g., between 1 mm and 25 mm inside diameter, or larger). For example, particles 50 having a diameter of 3 mm may work well with an acceleration nozzle 68 having an inside diameter of 6 mm, 25 mm, etc. As yet another example, particles 50 having a diameter of 6 mm may work well with an acceleration nozzle 68 having an inside diameter of 12 mm or larger.

The foregoing described embodiments depict different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.).

It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). 

What is claimed is:
 1. A peening system for increasing residual stresses in a target material, the peening system comprising: a liquid pump configured for pressurizing a liquid; a solid particles storage container configured for storing a quantity of solid particles; a peening head comprising: a liquid input port couplable with the liquid pump configured for receiving the pressurized liquid from the liquid pump; a solid particles input port couplable with the solid particles storage container configured for receiving the solid particles from the solid particle storage container; a liquid nozzle coupled to the liquid input port and configured for accelerating the pressurized liquid into a high velocity liquid jet; a mixing chamber coupled to the liquid nozzle and the solid particles input port, such that the solid particles are combined with the high velocity liquid jet and accelerated thereby in the mixing chamber; and an output nozzle coupled to the mixing chamber configured to accelerate the velocity of the liquid jet and solid particles mixture to permit the solid particles to impact a surface of the target material disposed at a stand-off distance from the output nozzle.
 2. The peening system of claim 1, wherein the liquid is housed in the liquid pump and comprises liquid water.
 3. The peening system of claim 1, wherein the liquid is housed in the liquid pump and comprises a cryogenic liquid.
 4. The peening system of claim 1, further comprising a robotic manipulator coupled to at least one of the peening head and the target material configured to selectively provide relative motion between the peening head and the target material in order to impart solid particle impacts over a desired area of the surface of the target material.
 5. The peening system of claim 4, further comprising a computer control unit operative to selectively control the movement of the robotic manipulator according to pre-programmed instructions.
 6. The peening system of claim 1, wherein the solid particles have a hardness that is less than the hardness of the target material.
 7. The peening system of claim 1, wherein the solid particles have a hardness equal to or less than 75% of the hardness of the target material.
 8. The peening system of claim 1, wherein the solid particles are made from a polymer material
 9. The peening system of claim 8, wherein the solid particles are made from rubber, acrylic, Viton®, or polyethylene.
 10. The peening system of claim 1, wherein the solid particles are made from brass, copper, lead, or aluminum.
 11. The peening system of claim 1, wherein the solid particles are made from an organic material.
 12. The peening system of claim 11, wherein the solid particles are made from corn husks or nut shells.
 13. The peening system of claim 1, wherein the mixing chamber is configured to draw the solid particles from the solid particles storage container into the mixing chamber by a vacuum created by the high velocity liquid jet passing therethrough.
 14. The peening system of claim 1, wherein the solid particles are accelerated by the high velocity liquid jet such that the solid particles impact the surface of the target material with a sufficient velocity to generate residual stress in the target material.
 15. The peening system of claim 1, further comprising a first conduit operative to couple the liquid pump with the liquid input port of the peening head and a second conduit operative to couple the solid particles storage container with the solid particles input port of the peening head.
 16. The peening system of claim 1, wherein the solid particles have a largest dimension that is between 0.025 mm and 1 mm.
 17. The peening system of claim 1, further comprising a control valve coupled to the solid particles storage container operative to regulate the mass flow rate of the solid particles flowing from the solid particles storage container to the solid particles input port.
 18. The peening system of claim 17, further comprising a computer control unit operatively coupled to the control valve configured to selectively control the operation of the control valve.
 19. The peening system of claim 1, wherein the liquid pump is configured to pressurize the liquid to a pressure greater than 10,000 pounds per square inch (PSI).
 20. The peening system of claim 1, further comprising a robotic manipulator coupled to at least one of the peening head and the target material configured to selectively provide relative motion between the peening head and the target material in order to impart solid particle impacts over a desired area of the surface of the target material, wherein the robotic manipulator is configured to maintain the stand-off distance between ⅛ of an inch and 15 inches.
 21. The peening system of claim 1, wherein the solid particles are accelerated such that the solid particles impact the surface of the target material at a velocity of at least 300 meters per second.
 22. A peening system for increasing residual stresses in a target material, the peening system comprising: a liquid pump configured for pressurizing a liquid to a pressure of at least 10,000 pounds per square inch (PSI); a solid particles storage container configured for storing a quantity of solid particles having a hardness of no more than 75% of the hardness of the target material; a peening head comprising: a liquid input port couplable with the liquid pump configured for receiving the liquid from the liquid pump; a solid particles input port couplable with the solid particles storage container configured for receiving the solid particles from the solid particle storage container; a liquid nozzle coupled to the liquid input port and configured for accelerating the liquid into a high velocity liquid jet; a mixing chamber coupled to the liquid nozzle and the solid particles input port such that the solid particles are combined with the high velocity liquid jet and accelerated thereby in the mixing chamber, the mixing chamber being configured to draw the solid particles into the mixing chamber by a vacuum created by the high velocity liquid jet passing therethrough; and an output nozzle coupled to the mixing chamber configured to accelerate the velocity of the liquid and the soft particles to permit the solid particles to impact a surface of the target material disposed at a stand-off distance from the output nozzle.
 23. A method of peening a target material to increase beneficial residual stresses therein, the method comprising: providing a quantity of solid particles; pressurizing a liquid; forming a high velocity liquid jet from the pressurized liquid; and accelerating the solid particles using the high velocity liquid jet such that the solid particles impact a surface of the target material to increase beneficial residual stresses therein.
 24. The method of claim 23, where the liquid comprises liquid water.
 25. The method of claim 23, where the liquid comprises a cryogenic liquid.
 26. The method of claim 23, wherein the solid particles are accelerated within a peening head, the method further comprising selectively moving at least one of the peening head and the target material relative to each other to impart solid particle impacts over a desired area of the surface of the target material.
 27. The method of claim 26, wherein the selectively moving is performed by a programmable robotic manipulator.
 28. The method of claim 23, wherein the solid particles have a hardness equal to or less than 75% of the hardness of the target material.
 29. The method of claim 23, wherein the solid particles are made from a polymer material, metal material, or organic material.
 30. The method of claim 23, wherein accelerating the solid particles comprises drawing the solid particles into a mixing chamber by a vacuum created by the high velocity liquid jet passing therethrough.
 31. The method of claim 23, wherein the solid particles have a largest dimension that is between 0.025 mm and 12 mm.
 32. The method of claim 23, further comprising selectively regulating a mass flow rate of the solid particles prior to being accelerated by the high velocity liquid jet.
 33. The method of claim 23, wherein pressurizing the liquid comprises raising the pressure of the liquid to a pressure greater than 10,000 pounds per square inch (PSI).
 34. The method of claim 23, wherein accelerating the solid particles comprises accelerating the solid particles to a velocity of at least 300 meters per second. 